Molar mass distributions of end-linked polystyrene star macromolecules

Polystyrene three-arm star molecules with OH tips were prepared by living anionic polymerization using an acetal-protected lithium initiator followed by coupling with 1,3,5-triallyl-oxy-2,4,6-triazine and ensuing cleavage of the acetol groups. The functionalized star molecules were end-crosslinked by toluene-2,4-diisocyanate. Randomly branched samples with various extents of crosslinking were prepared under conditions of full conversion of the isocyanate groups. The samples were fractionated analytically by SEC in on-line combination with LALLS and a viscosity detector. The mass fraction molar mass distributions could be well fitted by the Flory–Stockmayer distribution if the degree of crosslinking was X≤6. A broad region of crossover to percolation behaviour followed and the system reached its asymptotic region for X>200. Power-law behaviour was already obtained at X≈70, but the exponent τexp= 2·36 was found to lie between τFS=2·5 for the mean field approximation and τperc=2·2 for the percolation prediction.